1. Field of the Invention
The present invention relates to an air flow meter for measuring an air flow rate, and particularly relates to a heating resistor type air flow rate measuring device which is preferable for measurement of an intake air flow rate of an internal combustion engine of an automobile.
2. Background Art
Conventionally, heat resistor type air flow rate measuring devices are mounted in intake pipes of an automobile, a motorcycle and the like, detect intake air amounts, and send signals of them to ECU (Engine Control Unit).
When a contaminant such as exhaust gas which is emitted from an engine or other vehicles is taken into an intake pipe, the heat resistor of a heat resistor type air flow rate measuring device is contaminated by the contaminant. Further, when the intake pipe takes in water droplets or the like which are raised by a vehicle traveling ahead during drive in the rain or the like, the water droplets or the like adhere to the air cleaner element, and are further taken into the intake pipe, and the water drops comes flying to the heat resistor of the heat resistor type air flow rate measuring device. When the heat resistor is contaminated, the heat conductivity of the surface of the heat resistor changes, and differs in the heat radiation characteristic from the initial state (at the time of shipment), whereby even if the same flow rate contacts the heat resistor, the output value differs from the output value in the initial state, and an error occurs. When water droplets come flying, and the water droplets adhere to the heat resistor, the output waveform becomes spike-shaped due to the heat of vaporization, and correct output cannot be obtained until the water droplets disappear from the sensor element by being vaporized or passing it.
As the countermeasure against this, JP Patent Publication (Kohyo) No. 2002-506528 describes the device in which an auxiliary passage shape at the upstream side of the heat resistor which is to be a sensing part is made a detoured shape or spiral shape. In the device, when a contaminant or a water droplet comes flying from the upstream side of the air flow meter, the contaminant or the water droplet is separated from clean air by the centrifugal force which is brought about by the auxiliary passage shape in the detoured or spiral shape to avoid the heat resistor in the auxiliary passage.